An a-Si TFT sensor array is operated with the photosensitive characteristic of the amorphous silicon thin film transistors. There exist two kinds of the a-Si TFT sensor arrays: a charge-type sensor array and a current-type sensor array.
Please refer to FIG. 1, which is a circuit diagram showing a sensor element of a charge-type sensor array according to the prior art. The sensor element 1 includes a photosensing device 10, a storage capacitor 11 and a readout switch device 12. The photosensing device 10 generates a photocurrent in response to received light. The gate electrode 101 and the source electrode 103 of the photosensing device 10 are both coupled to a bias voltage 13 which is usually connected to common voltage. The source and drain electrodes 103, 102 of the photosensing device 10 are also coupled to the storage capacitor 11 which is discharged when the photosensing device 10 is exposed to light. The storage capacitor 11 is coupled to the source electrode 121 of the readout switch device 12, too. The charge on the storage capacitor 11 is read out periodically through the readout switch device 12 and a readout line 14. As shown, the gate electrode 122 of the readout switch device 12 is coupled to a switch line 15 to enable the readout switch device 12 switching. The drain electrode 123 of the readout switch device 12 is coupled to the readout line 14 to readout the charge.
Please refer to FIG. 2, which is a circuit diagram showing a sensor element of a current-type sensor array according to the prior art. The sensor element 2 includes a photosensing device 20 and a readout switch device 22. The drain electrode and the gate electrode of the photosensing device 20 are coupled to a bias voltage 23. Besides, the drain electrode of the readout switch device 22 is coupled to the source electrode of the photosensing device 20 and the source electrode of the readout switch device 22 is coupled to a readout line 24. The gate electrode of the readout switch device 22 is coupled to a switch line 25. Accordingly, the current of the photosensing device 20 is read out periodically through the readout line 24.
Since the compatibility with the manufacturing process of an LCD, the sensor element 1 or 2 can also be embedded in TFT-LCD as an input display for detecting light. Please refer to FIG. 3, which is a partial cross-sectional view showing a TFT-LCD embedded by sensor elements according to the prior art. As shown, the TFT-LCD 3 includes two substrates 30, 31, a liquid crystal layer 37, a color filter 32, a black matrix 33, readout switch devices 35 and photosensing devices 36. The photosensing devices 36 receive light passing through openings 34 and operate as the aforementioned descriptions.
Next, a current-type sensor array is taken for example to explain its operation principles. Please refer to FIG. 4, which is a partial circuit diagram showing a current-type sensor array 4 according to the prior art. The sensor array 4 includes m sensor elements 2 in each row and n sensor elements 2 in each column. Besides, there are m readout lines RO1-m and n switch lines SW1-n coupled to these sensor elements. The switch lines SW1-n are turned on one by one to reach the location in Y-direction and then the photocurrent is read out to reach the location in X-direction, so the two dimensional detection is accomplished.
Please refer to FIG. 5, which is a timing diagram showing the operation of the sensor array in FIG. 4. As shown, a photocurrent occurs on the readout line RO1 when the switch line SW1 is turned on, then the photocurrent is read out during the selection signal SL1 turned on. In other words, each selection signal corresponds to its corresponded readout line, for instance, the selection signal SL2 corresponds to the readout line RO2 and the selection signal SLm corresponds to the readout line ROm. It is noticed that a sudden high photocurrent appears in a short transient time a when the switch line SW1 begins to be turned on, so the unstable photocurrent is not read out from the readout line RO1. After the transient time α, the photocurrent becomes stable for being able to read out form the readout line RO1, and the period used to be read out the stable photocurrent is called readout time β.
The transient state of the photocurrent is caused by RC delay of the sensor element circuit itself and deep trap of the amorphous silicon. Please refer to FIG. 6, which is a timing diagram showing the variation of different photocurrent signals of FIG. 5. In FIG. 6, different photocurrent signal curves represent ones due to the different amounts of the received light. In other words, when the sensor element detects or receives a light which the unit of the light strength is lux, a photocurrent Iphoto occurs in the sensor element. Unfortunately, the photocurrent Iphoto is not always stable, and the amount of the photocurrent is a function of time.
As shown, a peak of the photocurrent signal appears in the transient time and then declines to a steady state. In the steady state, the photocurrent signal is readable. However, the transient time and the readout time are both affected by resolution. If the resolution is increased, the readout time will be reduced and the transient time will be raised. That means there may be no efficient time to read out the photocurrent. According to that, the resolution is limited, or the readout time cannot be easily extended.